Solid-state NMR spectroscopy is a powerful technique for the analysis of solids and semi-solids. It is a non-destructive and non-invasive technique that can provide selective, quantitative, and structural information about the sample being analyzed.
Maximizing the utility and increasing sensitivity and sample throughput for the analysis of materials using solid-state NMR spectroscopy is of interest because for most solid samples less than one percent of the time in the magnetic field is spent on data acquisition. The rest of the time (>99%) is spent waiting for the spin populations to return to their equilibrium value via spin-lattice relaxation (T1). However, the spin-spin relaxation time, T2, is usually several orders of magnitude shorter than T1. This means that the preparation and acquisition time in a Fourier Transform solid-state NMR experiment is typically tens of milliseconds. Before the sample can be pulsed again, the sample must relax for several seconds (T1) to several hours as the bulk magnetization returns to its equilibrium value. During this time the sample must remain in a large static magnetic field, but is not required to be in a homogeneous magnetic field.
One example of compounds that have long T1 times is pharmaceutical compounds. New drug compounds often are poorly crystalline or even amorphous, have long relaxation times, and are present at low levels in a formulation. This creates a significant problem for analyzing these compounds using solid-state NMR spectroscopy, because analysis times can range from a few minutes to a few days depending upon the state of the sample (i.e. bulk drug or formulated product), relative sensitivity (i.e. choice and number of different nuclei in molecule), and relaxation parameters. For example, to quantify a mixture of two forms of a compound can take a few hours (for a sample with short relaxation times) to a few days. To analyze a series of formulated products may take a month or more of spectrometer time. This leads to low throughput, high cost per sample analysis, and has relegated solid-state NMR spectroscopy in many cases to be a prohibitively expensive problem-solving technique compared to other analytical techniques such as powder X-ray diffraction, infrared and Raman spectroscopy, and Differential Scanning Calorimetry (DSC).
Also, throughput has been a significant problem in NMR spectroscopy, because the design of the NMR magnet generally allows the analysis of only one sample at a time. Autosamplers have increased throughput by minimizing the time spent changing samples and by allowing continuous use of the spectrometer, but have not increased the number of samples that could be run if samples were changed promptly.
Some researchers have used strategies for the acquisition of multiple signals from multiple probes that are packed within the homogeneous portion of the magnet to maximize the utilization of an expensive analytical tool. For example, Oldfield, et al., A Multiple-Probe Strategy for Ultra-High-Field Nuclear Magnetic Resonance Spectroscopy, J. Mag. Res., Series A 107, 255-257 (1994), discloses the incorporation of more than one probe in the homogeneous part of the magnet that allows for the acquisition of a spectrum for each individual sample. Oldfield discloses a probe that contains three different samples which are all simultaneously located in the homogeneous part of the magnet. Although Oldfield disclosed that the samples were static, he proposed that at least one could incorporate sample spinning. The resolution of this system was quoted as approximately 1 ppm.
The concept in Oldfield was extended to solution NMR spectroscopy by Raftery and coworkers. U.S. Pat. Appl. Pub. No. 2002/0130661 A1. Raftery, et al., have shown that up to four different samples could be located simultaneously in the homogeneous part of the magnetic field. However, the larger the number of samples, the smaller the sample volume must be for all samples to be located simultaneously in the homogeneous region of the magnetic field. This design does not allow for the easy incorporation of magic-angle spinning (MAS) for multiple samples at typical (0.5 cm2) solid sample sizes.
Poor signal to noise ratio (SNR) is also a significant problem in the analysis of materials using solid-state NMR spectroscopy. The most common method used to increase the SNR in samples containing nuclei with low magnetogyric ratios, low natural abundance, and low sample concentration is signal averaging. The SNR in an NMR experiment is proportional to the signal divided by the noise. This relationship is as follows:   SNR  ∝                    [                              N            ⁢                                                   ⁢                          ξω                              3                /                2                                                          T                          3              /              2                                      ]            ⁡              [                              Q            ⁢                                                   ⁢                          V              c                                F                ]                    1      /      2                      where N=Nuclear spins per unit volume         ω=Larmor precession frequency         ξ=Filling factor of the receiver coil        °T=Temperature (absolute)         Q=Quality factor of the receiver coil         Vc=Volume enclosed by the receiver coil         F=Noise figure of the preamplifier        
This equation suggests two approaches to improve SNR: increase signal, or decrease noise (or both). The equation for the SNR in a Fourier Transform NMR experiment is shown. This equation assumes that the sample and coil are at the same temperature, and does not take into consideration fixed parameters such as linewidth, magnetogyric ratios, spin quantum numbers, etc. A further discussion of optimizing sensitivity can be found in Freeman. Freeman, R. A Handbook of Nuclear Magnetic Resonance; John Wiley and Sons Inc., New York, 1988.
Increasing the SNR by signal averaging is a problem if the sample has long spin-lattice relaxation times (T1) of minutes, hours, or even days, because the number of transients acquired is limited to one or at most several dozen. Table 1 shows the relaxation times for many of the pharmaceutical solids reported in the literature.
TABLE 1Relaxation times for various pharmaceutical compounds.1H ResonanceDelay at 400CompoundFrequencyRecycle DelayMHz (9.4 T)R.O.Y.*30040-70 s70-125 sCimetidine36015 s18 sLY2978024005-10 s5-10 sEphedrine2001.5 s6 sAspirin30090 s160 sSalicyclic Acid3001 hr1.78 hrsprednisolone t-2003 s12 sbutylacetateAcetaminophen2002 s8 sCarbamazepine2003 s12 sEnalapril Maleate2002 s8 sIbuprofen2002 s8 s*5-methyl-2-[(2-nitrophenyl)amino]-3-thiophenecarbonitrile 
For example, aspirin is a representative pharmaceutical solid that has a 1H T1 relaxation time of approximately 30 s at 300 MHz. In a 13C cross polarization magic-angle spinning (CPMAS) experiment the pulse delay between acquisitions must be at least 90 s to avoid saturation. With salicylic acid, which does not have a methyl group, the delay between acquisitions is greater than 1 hr. It should be noted that some of these compounds may have been chosen because they have relatively short relaxation times, and that the recycle delays may not have been optimized.
One potential solution to the sensitivity problem is to use to higher magnetic fields, but that has several significant disadvantages. First, resolution often will not increase dramatically at higher fields if the linewidth of the sample is limited by bulk magnetic susceptibility or a range of conformations. For example, most drug compounds are still relatively low molecular weight species (<500 MW). At field strengths as low as 7-9 Tesla there is sufficient resolution to identify most, if not all, of the peaks in the spectrum. Second, higher fields require faster spinning speeds to obtain the same separation in ppm between isotropic peaks and spinning sidebands. Increasing the magnetic field from 9.4 Tesla to 18.8 Tesla would require doubling the spinning rate, which usually corresponds to a decrease of at least a factor of two in sample volume. Third, for crystalline solids, especially those without methyl groups, the relaxation rate is often inversely proportional to the square of the magnetic field strength. Going from 7.05 T to 18.8 T would increase relaxation delays by about a factor of seven, mitigating significantly any increased sensitivity gains obtained by going to higher field strengths.
Sensitivity in a MAS experiment can also be improved by increasing the sample volume. However, there are several significant limitations to the development of a large MAS probe capable of cross polarization and high power 1H decoupling. They include:
(1) Producing a high-power 1H RF field capable of minimal decoupling for a moderately rigid proton environment (approximately 25 kHz);
(2) The ability to spin a large sample at the magic angle with minimal sidebands; and
(3) Producing a magnetic field of uniform homogeneity over the entire sample. Unfortunately, the methods needed to overcome each of these limitations are not trivial. For example, while magnetic resonance imaging (MRN) technology requires higher resolution magnetic fields over a much larger sample volume than is needed for solid-state NMR experiments, incorporating such a magnet into a pharmaceutical or chemical laboratory would be quite difficult, especially at high fields (>400 MHz). Large sample volumes inherently mean slower spinning speeds, which is also not desirable. Finally, the ability to adequately decouple the 1H nuclei from the X nuclei is extremely problematic, as it would require very high power 1H decoupling amplifiers (>4000 W).
Some research groups are currently developing methods to increase nuclear magnetic resonance sensitivity, using techniques such as hyperpolarized xenon or dynamic nuclear polarization (DNP). One recent method that demonstrates substantially increased sensitivity in solution is a cryoprobe that, at considerable expense, keeps the coil and preamplifier at close to liquid helium temperatures. The sensitivity gains occur because of lower noise figures for both the coil and the preamplifier, and a higher Q. Some sacrifice is made in filling factor, which limits the gains in sensitivity. In the solid state, especially for magic-angle spinning (MAS) systems, cooling the coil without cooling the sample would be extraordinarily difficult. Even cooling the system under MAS conditions is technically challenging.
In the solid state, other methods such as variable-amplitude cross polarization (VACP) and two-pulse phase modulation (TPPM) decoupling can improve the signal to noise ratio by approximately 30% for ideal samples. Similar gains can potentially be made by optimizing probe circuitry.
Also, the concept of moving samples into the homogeneous magnetic field only when the spectrum is being acquired has been used to measure relaxation times at different magnetic field strengths, and also to measure dipolar couplings at field strength of zero. In these approaches, the desire was usually to move the sample to a different magnetic field strengths, but not keep all of the samples always in the highest magnetic field strength.
Even with the advances described above, there have been no dramatic improvements in sensitivity and throughput for solid-state NMR spectroscopy for the routine analysis of low abundance nuclei such as 13C and 15N since the development of the cross polarization magic-angle spinning (CPMAS) experiment almost three decades ago. Further, none of the currently available techniques improve sensitivity and throughput by repositioning a probe within the bore of the magnet to analyze a different sample and allow the spins of one or more other samples to return to a state that allows a researcher to acquire another spectrum of the one or more other samples.